This section provides background information related to the present disclosure which is not necessarily prior art.
An LLC resonant converter having excellent efficiency characteristics is widely used as a direct current/direct current converter. However, a problem of ripple output may arise. Unlike high-frequency switching noises, ripples are within a low frequency range of a spectrum. In general, ripples come from an electrical grid, and a frequency of the ripples is twice as much as that of commercial electricity, approximately 100 Hz or 120 Hz. In most applications, the amplitude of output ripples needs to be limited, since unlimited ripples may cause unstable performance, abnormal operation or even damage of equipment.
In the prior art, by using a method of performing feedback control to increase an open-loop gain, output ripples of an LLC resonant converter are reduced to some degree. However, this method is not very effective in reducing ripples, because it is difficult to make a tradeoff between stability and a high gain.